1. Field of Invention
This invention relates generally to an industrial process control system in which an electronic controller compares a process variable signal with a set point signal to produce an output signal that adjusts a final control element regulating the process, and more particularly to a system of this type in which the controller is self-tuning and takes into account process "dead time."
2. Status of Prior Art
An electronic controller is a component in a process control closed loop that is subject to disturbances, the controller acting in conjunction with other devices to maintain a process variable at a desired value. The variable controlled may be be flow rate, pressure, viscosity, liquid level, or any other process variable. In operation, the electronic controller receives, in terms of corresponding input signals, both the process variable and a set point, and it compares these signals to produce an output signal that reflects the deviation of the process variable from the set point. This output signal, when applied to a final control element in the loop will directly or indirectly govern the process variable.
Thus one input signal to a controller may be derived from a flowmeter whose reading is converted into a corresponding electrical value, and the output signal may be impressed on a flow-regulating valve which is caused to assume an intermediate position between open and closed at which the flow rate conforms to the set point. The set point generator may be an internal component of the controller or a remotely-controlled device.
Variations in controller action are obtained by adjustment of parameters associated with the control modes and are available in several combinations. These modes of control action which are combined to adjust the controller output signal are known as proportional, reset and derivative.
Proportional action produces an output signal proportional to the deviation of the controlled process variable from the set point. The amount of deviation in terms of percentage required to move the final control element through the full range is known as the proportional band. Automatic reset action, also known as integral action, produces a corrective signal proportional to the time integral value of the deviation of the controlled process variable from the set point, while derivative action, also known as rate action, produces a corrective signal proportional to the rate at which the controlled variable is changing. Manual reset action is an operator-actuated potentiometer controlled to produce a corrective signal directly proportional to the magnitude of the adjustment.
The typical industrial process includes operating elements whose characteristics are such as to introduce delays or retards in the value of the process variable. These delays and retardations are termed process time lags and arise from capacitance, resistance and dead time effects.
To illustrate the nature of process time lags, we shall assume a controlled process in which water in a tank is heated by coils, the temperature of the water in the tank being sensed by an electronic thermocouple which generates a process variable signal. The temperature of process water in the tank which is raised by the steam flow supplied thereto is reduced by cold water fed into the tank. The steam flow is under the control of a controller. The controller compares the process variable signal from the thermocouple with a set point signal to produce an output signal that adjusts the steam valve to bring the temperature to the set point.
Because the walls of both the tank and the steam coils and the water in the tank are capable of storing heat energy, this energy-storing characteristic gives these parts in the process the capacity (C) to retard change. Those parts of the process which resist the transfer of energy between capacities are termed resistance (R). Thus since the coils carrying the steam are immersed in water, the walls of these coils and the insulating effect of the steam and water on either side of the walls resist the transfer of energy between the steam in the coils and the water outside the coils. The combined RC effect of supplying a capacity through a resistance constitutes a process lag time constant.
In the example given above, process water in the tank is heated by steam whose flow is controlled by the controller. However, some time will elapse before the temperature change caused by the steam flow adjustment reaches the thermocouple to cause a change in the process variable. This time lag is not just a slowing down of change but a discrete time delay or "dead time" during which no change whatever occurs. The length of the dead time depends on both the velocity with which this change is transported and the distance over which it is carried.
It is known in a process control system to provide a self-tuning controller which takes into account controlled parameters such as K.sub.p (process gain), T.sub.p (process time constant) and D (process dead time) and adjusts the controller parameters based on the identification of these parameters.
Thus in the article "Microprocessor-based Adaptive Speed and Position Control for Electrical Driver" which appears in Vol. 1A-21, No. 5, September/October 1985 issue of the IEEE Transactions on Industry Applications, a control system is disclosed in which a recursive algorithm is used to identity the controlled process.
It has been found that this known recursive algorithm applied to a self-tuning algorithm has distinct drawbacks; for if the present dead time D is a factor of two different from the actual dead time, the calculations for parameters K.sub.p and T.sub.p were then found to be quite different from the actual values; hence were unusable. Moreover, there were instances where the algorithm could not converge on parameter values.